A state of the art Electrical Capacitance Tomography system has been employed to interrogate a fluidised bed of 3 mm polyethylene granules in a 127 mm diameter bed. Two sets of 8 electrodes were mounted at positions up the bed from 150 to 650 mm from the distributor. The experiments have been carried out at sampling frequencies of 200-2000 cross-sections per second. Gas superficial velocities from just below the minimum fluidisation to 83% above minimum fluidisation velocities were used. Initially, times series of the cross-sectionally averaged void fractions have been examined both directly and in amplitude and frequency space. The last two used Probability Density Functions and Power Spectral Densities. The information generated show that the fluidised bed is operating in the slugging mode, not surprising given the sizes of the particles.

General Note:

The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows

Abstract
A state of the art Electrical Capacitance Tomography system has been employed to interrogate a fluidised bed of 3 mm
polyethylene granules in a 127 mm diameter bed. Two sets of 8 electrodes were mounted at positions up the bed from 150 to
650 mm from the distributor. The experiments have been carried out at sampling frequencies of 200-2000 cross-sections per
second. Gas superficial velocities from just below the minimum fluidisation to 83% above minimum fluidisation velocities
were used. Initially, times series of the cross-sectionally averaged void fractions have been examined both directly and in
amplitude and frequency space. The last two used Probability Density Functions and Power Spectral Densities. The
information generated show that the fluidised bed is operating in the slugging mode, not surprising given the sizes of the
particles.

Introduction
Fluidised beds are found in may industrial applications.
These range from the drying of polymer granules to
eliminate residual water to the gasifier units of Integrated
Gasifier Combined Cycle power generation plant. These
new generation plants, particularly if oxy-fired, will make
it simpler and more efficient to implement Carbon Capture
and Sequestration measures. The present work is part of
research in this latter area. In an earlier project,
Azzopardi et al. (2008), Electrical Capacitance
Tomography was used to monitor the feed rates of the
pulverised fuel into the gasifier under dense-phase
pneumatic conveying conditions. In the current work the
behaviour of fluidised bed gasifiers is being investigated.
This paper report on our commissioning work which has
used larger polyethylene pellets (3 mm) rather than the
finer pulverised coal (60 Vpm) more commonly used in the
gasifier.

In order to analyse the hydrodynamic behaviour of
fluidised beds, several studies have been carried out.
Most of them concentrated on the minimum fluidisation
velocity of the bed as it is one of the key parameters in the
design of fluidised beds, e.g., Hilal et al. (2001) who
showed the effects of the bed diameters, distributor and
inserts on minimum fluidisation velocity. The correlations
proposed for calculating this minimum velocity are
numerous, some of them are reported by Thonglimp et al.
(1984); for example the early work of Wen and Yu (1966).
Several other papers have emphasized the bed expansions,
e.g., Hilal and Gunn (2002) and Thonglimp et al. (1984).

Geldart (1973, 1978) using his data and those from
literature described the fluidisation characteristics of
powders by the mean size of the powder and the density of
the particles. He defined thus four groups, A (Aeratable), B

(Sand-like), C (Cohesive) and D (Spoutable). This
representation is usually referred as the Geldart
classification.

The efficiency of fluidised bed depends greatly on the flow
pattern. Beck et al. (1998) presented a state of art review
on tomographic methods and their particulate attributes
(speed, sensitivity, robustness, etc.) used for analysing the
process behaviour. They focused more particularly on the
electrical ones.

In the 1970s, work on a capacitance measuring system was
initiated at the Morgantown Energy Technology Center
(METC) to measure levels of beds, solid flowrates and
gas-solid mixtures parameters, Halow et al. (1992). This
work led to a non intrusive system allowing three
dimensional imaging of the voidage distribution in
fluidised beds at rates of 60 to 100 frames per second.
Halow et al. (1992) reported observations of the flow in
fluidized bed using (193 pixel) resolution system. The bed
was imaged in zone 1.25 to 2 bed diameters above the grid,
at a rate of 62.5 frames per second for a period of 5.23 s.
Halow et al. (1993) performed imaging in a region 3 to
3.75 bed diameters above the grid, at a rates of 62.5 frames
per second for a period of 20.9 s.

Wang et al. (1995) used the ECT technique to analyse the
flow pattern close to the air distributor of a fluidised bed.
They used the UMIST-type sensor (sensor developed at the
University of Manchester Institute of Science and
Technology). Imaging at a frame rate of 210 Hz and 812
pixel resolution was achieved. The results presented
concerned three flow regimes: bubbling fluidisation,
slugging and transition from slugging to a turbulent
regime.

Kiihn et al. (1996) analysed the chaos in fluidisation. They
used the ECT to measure the local porosity of gas-solids
fluidised bed as well as its time fluctuations. They
employed the METC-type sensor. The authors noticed that
this sensor gives reliable results on large scale (28.4 cm
diameter) fluidised bed.

Sidorenko & Rhodes ('i"'14) have employed Electrical
Capacitance Tomography to study the effect of pressure on
the behaviour of fluidised beds of two materials FCC
catalyst (77gm) and silica sand (203 jm). These are
classes A and B respectively under the classification of
Geldart (1973). They found little effect of pressure on the
minimum fluidisation velocity. For the silica sand the
void fraction increased linearly with gas superficial
velocity and the characteristic frequency of the fluctuations
in void fraction were seen to be almost independent of gas
superficial velocity. The trends for the FCC catalyst were
found to be more complex.

Experimental Facility
The experiments were carried out on a vertical 127 mm
diameter 4 m long cylindrical column fed by air from the
laboratory compressed air main. The column is made up of
transparent acrylic resin pipe. At the bottom of the column
there is an air distributor which consists of a 3 mm thick
porous plate made of plastic with a maximum porous size of
50-70 pm. Below this there is a conical section filled with
6 mm diameter glass beads to create an even distribution of
the flow. Air is provided from the laboratory compressed
air main Its flow rate is measured using a calibrated
variable area meter.

The bed material is polyethylene in the form of granules of
nearly spherical shape approximately 3mm in diameter.
This material has a density of 915 kg m3 and a bulk
density of 558 kg m37 which corresponds to an as-poured
void fraction of 0.39. These particles are class D in the
classification proposed by Geldart (1973) as shown in
Figure 2.

Electrical Capacitance Tomography
Instrumentation
The Tomoflow R5000 is a high-speed imaging system
which can be used for imaging and velocity measurement
in flows of mixtures of 2 non-conducting materials.
Using electrical capacitance tomography (ECT) can offer
measurements unobtainable with other measurement
technologies, but the interpretation of quantitative flow
data requires a good physical model of the interaction of
the materials with the electric field in the sensor and
appropriate reconstruction and analysis algorithms.

Most ECT sensors are non-linear, both in the relationship
between the measured capacitances and the permittivity of
the sensor contents and also in the relationship between the
concentration of a 2-phase mixture and its effective
permittivity. Linearisation algorithms were applied to the
measurements before any images were reconstructed.

Data can be captured at rates up to 5000 image frames per
second with typical measurement noise level at 500 fps of
0.02fF rms. The typical average value between two
opposite electrodes is 10fF. In the experiments reported
here the frame rate was between 200 and 1000
cross-sections per second.

Paper No

D
B
A

C

Paper No

The instrument contains 16 identical measurement
channels and 16 identical driven guard circuits and in the
tests reported here was operated with a twin-plane sensor
containing 8 azimuthal electrodes. The sensor included a
full set of driven guard electrodes running axially before,
between and after the measurement planes ensuring that an
axially-uniform electric field was maintained over the
capacitance sensor cross-section

Measurements were made between all pairs of electrodes
within each plane around the sensor using a
charge/discharge capacitance technique. An excitation
signal was used in the form of a 15V peak to peak square
wave with a frequency of 5 MHz.

Inversion of the 28 capacitance pairs to a 812 pixel image on
a 32x32 square grid was undertaken using a technique
known as linear back projection, and component
information (void fraction etc.) is extracted from these
images. Cross-correlation between the image planes gives
the velocity distribution across the flow.

The two electrode rings were positioned with their
centerlines 150 and 650 mm from the distributor plate.
Some experiments were also carried out with the lower ring
300 mm from the distributor plate.

Results and Discussion

The minimum superficial velocity was determined to be
0.789 m s-' from a plot of pressure drop across the bed
against the gas superficial velocity. This compares well
with values calculated from correlations in the literature
(within 15%), e.g., the equation of Hilal et al. (2001).
As the fluidised bed was made from transparent acrylic
resin pipe, it was possible to observe visually the bed
behaviour. Below the minimum fluidisation velocity
there is hardly any motion in the bed. However, at higher
velocities there are sudden expansions of the bed with
large plug of particles being pushed upwards. Individual
particles rain down from these plugs.
Measurements were made at eight gas superficial velocities,
three below and five above the minimum fluidisation
velocity. Tests were carried out at sampling frequencies
of 200, 1000 and 2000 cross-sections per second.
The output of the Electrical Capacitance Tomography can
be examined at several levels. The simplest is to obtain
time series of the cross-sectionally averaged void fraction.
Examples of these are shown in Figures 3-6.

Data for a gas velocity 1.16x the minimum fluidisation
velocity is shown in Figure 3. At the lower plane,
approximately one bed diameter from the gas distributor, it
can be seen that there is a number of small increases in
void fraction from the packed bed value. These occur at
about once per second and do not get to a void fraction of
1.0, i.e., they do not total fill the entire bed with gas.
Examination of the cross-sectionally resolved information
shows gas bubbles occupying about one third of the bed
cross-section. At the higher plane the bubbles have
increased in size and do occupy most of the cross-section.
However, the time for which they exist varies from bubble
to bubble. At 1.33x minimum fluidisation velocity there
are some bubbles at the lower plane which fill the entire
cross-section though there are others which are still
collections of smaller bubbles as seen in Figure 4. At the
upper plane they are on the whole large bubbles.
However, the peak void fraction is lower than the
corresponding values in Figure 3. This is probably
caused by the bubble breaking up by particles raining
down from the packed bed above. By 1.5x minimum
fluidisation velocity the large bubbles are well formed at

Paper No

the lower plane where they essentially fill the cross-section.
Figure 5 illustrates how at the upper plane the shape is
similar but with lower peak void fractions. When the gas
velocity is 1.67x the minimum fluidisation velocity there
are still the large bubbles at the lower plane but in some
cases there are collections of smaller bubbles. Figure 6
shows how they are all large bubbles at the upper plane.

The time averaged void fractions are shown in Figure 7
where it is seen that the sampling rate has only a small
effect on the results. Obviously below the minimum
fluidisation velocity there is hardly any effect of gas flow
rate. Beyond this value the void fraction increases with
gas superficial velocity. However, eventually, it plateaus
out to a constant value.

Density Functions (PDF), a measure of how often each
void fraction occurs. Figure 9 presents the PDFs for one
gas flow rate and considers the differences between the
two axial positions. At the lower plane, there is a range
of void fractions present. This probably indicated smaller
bubbles which are coalescing to form larger one.
However, by the higher plane large bubbles have been
formed and the flow consists of alternate zones of gas and
packed beds.

If the effect of gas superficial velocity is considered,
Figure 10, it is seen that there is hardly any change in the
void fraction of the packed bed zone. Again at just above
minimum fluidisation velocity, there is evidence of just
small bubbles. As the gas flow rate is increased it can be
seen that the void fraction at which the second, higher peak
occurs moves to higher values showing that the size of the
large bubble is increasing.

Fourier transform of the auto covariance functions. Since
the auto covariance function has no phase lag, a discrete
cosine transform can be applied.

The auto covariance function of a signal x(t) is given by:

RX(kA)= [x(t)-x][x(t+kAr)-x] dt ; T< T

(1)
where T is the sampling duration, kAr is the time delay, r

is the interrogating time delay and x = x(t) dt.

The Power Spectrum Density is then obtained from:

P,(f)= -Rx (0) + ZR,(kAr)w(kAr)cos(2kAir)J
2 k=-
(2)
where w(kAr) is a windowing function. Windowing
functions help to suppress the spectrum leakage which
mostly comes out as the side lobes at the high frequency
end of the spectrum. By using appropriate windowing
function the frequencies contributing the system becomes
clear. In initial analysis carried out here, a basic cosine
windowing function was used,

w(kA r) = cos -k (3)

Figure 11 shows examples of the Power Spectral density for
void fraction taken at plane 1 for a range of gas superficial
velocities. These all show a clear peak, the most likely
frequency.

The time series data presented in Figures 3-6 can be
analysed further to yield the velocities of the large
bubbles/packed bed zones. By selecting values of void
fraction to be the transition between one and another zone,
the times at which the transitions occur can be extracted.
From these the velocities of the fronts and backs of the
bubbles are calculated. Figure 13 illustrates that though
there is scatter of individual values, there is no obvious
trend with time. The mean values of the velocities of the
front of the bubbles are 0.53 of the gas superficial
velocities for the data hitherto analysed (1.18 and 1.32
m/s).

1
Frequency (Hz)

0 2 4 6 8
Time (s)

10 12 14

Figure 11: Power Spectral Densities for
Sampling frequency = 2000 Hz.

plane 1.

Similar curves are obtained from the data taken at plane 2.
Here there is less variation in the position of the peak.
The frequency at which the peak occurs can be taken as the
most probable frequency. The values obtained for both
planes are plotted in Figure 12 which shows that there is
hardly any variation with gas superficial velocity. A
similar result had been reported by Sidorenko & Rhodes.
However, they measured a frequency of ~3 Hz. It is
noted that they worked with small particles silica sand of

Form the above it can be concluded that the Electrical
Capacitance Technique employed has great capability for
interrogating fluidised beds. Even from just the
cross-sectionally averaged time series of void fraction, there
are many features of the flow that can be identified.